The present invention relates to contact sensors for calipering pieces such as workpieces.
Such sensors are incorporated in calipering apparatus which are frequently employed with machine-tools for controlling the dimensions of a workpiece with great precision during or at the end of machining.
More specifically, the invention concerns a sensor of the type comprising:
a feeler;
a suspension device comprising at least one first part to which the feeler is rigidly connected and a second part assembled for movement relative to one another in a first direction;
a transducer for measuring the amplitude of the relative movement of the first and second parts; and
force generating means for holding the feeler in a stable rest position and allowing the feeler to exert on the surface of a piece to be calipered a bearing force which initially increases linearly and rapidly as the feeler moves away from its rest position in a calipering zone and which then increases at a slight rate in a disengagement zone, this force generating means comprising first elastic means connected to one of said parts of the suspension device and deformable in said first direction and an assembly carried by the other part of the suspension device which assembly comprises abutment means, two mobile elements which interact with the first elastic means, and second elastic means acting oppositely on these mobile elements to permanently bias them into abutting engagement with the abutment means, the first and second elastic means being such that, in the calipering zone, the mobile elements remain in contact with the abutment means to cause the first elastic means to deform and, in the disengagement zone, one of the mobile elements progressively moves away from the abutment means under the action of the first elastic means and against the action of the second elastic means which in turn are deformed.
A known sensor of this type is described in U.S. Pat. No. 3 945 124 and is shown schematically in longitudinal cross-section in FIGS. 1 and 2 of the accompanying drawings, FIG. 2 being a cross-section along line II--II of FIG. 1.
In this known sensor, the two above-mentioned assembled parts of the suspension device are constituted by two rigid plates 2 and 4, plate 2 carrying a feeler point 6. The plates 2 and 4 are connected by two elastic blades 8 and 10 so as to form a deformable parallelepiped which allows the feeler point 6 to move relative to the plate 4 (with which it is not rigidly connected) only in a direction parallel to this plate as indicated in FIG. 1 by the opposite arrows f and f'.
To simplify the drawing, the transducer e.g. of the capacitance or inductance type, for measuring the amplitude of movement of the feeler point, is not shown.
However, these Figures show the force generating means comprising a cage 12 fixed to the plate 4 and separated in two by a median wall 14 having therein recesses housing three balls 16 arranged around a circle. The diameter of balls 16 is slightly greater than the thickness of wall 14.
On either side of the wall 14 are two flanges 18 and 20 on the ends of rigid guide rods 22 and 24 slidably mounted in the cage 12 and biased by identical pre-stressed coil springs 26 and 28 to permanently hold the flanges 18 and 20 against the balls 16.
Finally, in this instance the force generating means also comprise a flexible rod 30 for example made at least partly of piezo-electric material. One of the ends of rod 30 is solidly fixed to the plate 2 and its other end carries a spherical head 32 of the same diameter as the balls 16. This head 32 is received in a fourth recess of the wall 14 and is located practically at the center of the circle on which the balls 16 are arranged.
Hence, as long as the feeler point 6 is not subjected to any action which makes it move relative to the plate 4 in the direction of arrows f and f', it remains in a rest position which serves as "zero" or reference position for the measurements, in which rest position the elastic blades 8, 10 and rod 30 of course are not deformed. In this state, which is shown in FIGS. 1 and 2, the head 32 of rod 30 exerts no force on the flanges 18 and 20 so that these flanges are in contact with the balls 16.
Suppose the plate 4 is fixed and a workpiece to be calipered is brought into contact with the feeler point 6 in the direction of arrow f'. At the moment of contact, the feeler point 6 is still in the rest position and exerts no bearing force against the workpiece.
If the workpiece is then further moved in the direction of arrow f', the elastic blades 8 and 10 bend and the flexible rod 30 also bends until the force exerted by the head 32 on the flange 18 equals the force of the spring 26. From then on, the rod 30 practically does not bend any more and the spring 26 is compressed along with further bending of the elastic blades 8, 10.
Of course, if the workpiece were brought in the direction of arrow f into contact with the feeler point 6 and continued to move in this direction, the blades 8, 10 and rod 30 would bend in the opposite direction and the flange 20 would be pushed against the spring 28.
Referring now to FIG. 3 which graphically illustrates the variation of the bearing force exerted by the feeler point 6 on the workpiece as a function of its movement relative to the plate 4, it can be seen that initially the force increases from zero linearly and rapidly as the rod 30 bends and then continues to increase linearly but much slower from the moment when the rod 30 moves one of the flanges 18 or 20 against the action of the spring associated therewith.
As already indicated, the zone in which deflection of the feeler corresponds to a rapidly increasing bearing force is where calipering measurements can take place. The other so-called "disengagement" zone is much larger and is provided for the purpose of preventing damage to the sensor and/or the workpiece during calipering. For example, it can brake and possibly stop the forward feeding of a workpiece on a machine-tool when the workpiece approaches the sensor or vice-versa.
The validity of the above discussion concerning the bearing force is based on the assumption that the balls 16 and head 32 have exactly the same diameter.
If, however, the diameter of the head 32 is slightly less than that of the balls 16, when the feeler point 6 is deflected the rod 30 does not bend immediately, but only when the head 32 comes into contact with one of the flanges. Consequently the measuring force, i.e. the force with which the feeler bears on the workpiece being calipered, results initially only from bending of the elastic blades 8 and 10 and is therefore practically zero as shown by curve A of the graph of FIG. 4 which is shown on a much bigger scale than FIG. 3.
If, however, the diameter of the head 32 is greater than that of the balls 16, the head 32 is from the beginning acted upon by the oppositely-acting springs 26 and 28 one of which allows the rod 30 to follow movement of the feeler against the action of the other. Because of this, the calipering force initially increases only very slowly. From the moment when the flange 18, if the feeler moves in the direction of arrow f', or the flange 20, if the feeler moves in the direction f, contacts the balls 16, this force begins to increase rapidly. This is illustrated by curve B of FIG. 4 .
Hence, in both cases the rest position of the feeler is no longer well defined. This may result in a hysteresis phenomenon sufficient to disturb the accuracy of calipering if the difference between the diameter of the head of the rod and the diameter of the balls is of the same order of magnitude as the resolution of the sensor.
Consequently, if as is frequently the case the sensor should be able to measure to an accuracy of one tenth of a micron, the manufacturing tolerances for the balls and the head of the rod are so small that they are virtually zero. This evidently is a problem.
Moreover, during use of the sensor the balls do not wear at the same rate as the head of the rod which means that even if the problem of achieving the manufacturing tolerances has been overcome, there still remains a possibility of errors in the accuracy of calipering.
Finally, this type of sensor has a further defect, namely that for large deflections of the feeler the head of the rod slides and rubs against one of the flanges. This can also produce hysteresis and for this reason the calipering zone is necessarily of very limited dimensions. For example, with the sensors currently commercialized by Messrs. Ernst Leitz GmbH, proprietor of the cited US patent, calipering can only take place in a zone of +/- 16 micron. However, for some applications it would be advantageous to have a much larger calipering zone.